Typically a grid of power lines distributes electricity from either the point of generation or substations throughout both metropolitan and rural areas. Power is normally distributed at 11-36KV from the point of generation or from substations. In order to effectively supply power (electricity) for domestic use in home or business, transformers are employed at substations so as to step down the voltage to lower levels for subsequent conducting through a distribution network. Such transformers typically supply domestic users with between 110 to 450 V of electricity. Therefore transformers are subject to high input voltages and lower output voltages which supply a subsequent power distribution network.
The transformer requires insulative conductors on both the high and low voltage sides to protect the transformer from shorting to the Earth. The transformer case is usually metal and must be safely insulated. Electrical bushings provide a safe means of passing electrical current through the transformer casing. Normally an electrical conductor is sealed within such an electrical bushing so as to prevent leakage between the bushing and the conductor and to present electrical terminals extending from the interior of the transformer tank.
Electrical bushings of the prior art are produced in a variety of configurations so that they can be permanently affixed upon an electrical transformer by a welding process or the like. The bushing presents an electrical conductor through an opening in the wall of a transformer into the interior thereof which is filled with transformer oil or inert gas. Conventional bushings are provided with an insulative component and an exterior mounting flange affixed upon the component, which flange is adapted to be welded upon the transformer tank in order to permanently fix the bushing to the transformer in a way which seals the opening therein. It is a requirement that the bushing provides a mechanical seal, air to air, air to gas, or air to oil, dependent on the transformer design.
In the prior art, bushings welded upon a transformer suffer the disadvantage of sacrificing the ability of the bushing or transformer to be easily repaired or replaced. It is a frequent occurrence, due to accident or malfunction, that the bushing requires repair or replacement, however because the bushing can't be easily removed without extensive labour, complete replacement of the transformer is often the end result.
Further disadvantages of the prior art such as oil leakage, may occur because the welding process to seal the bushing into the transformer may not form a complete seal, it is often the case that such leakage will not be detected until final stages of manufacture when corrective measures are costly.
Generally the prior art can be regarded in two categories—ceramic and non-ceramic. Ceramic prior art has been limited in design due to the materials employed. For example porcelain bushings cannot be cast in asymmetrical shapes. Also, the difference in expansion coefficient between the conductor and the insulator means that the conductor can never be cast into the bushing. This limitation complicates both the assembly of the unit as a whole, the fixing of the bushing to the transformer and also the sealing of the bushing unit.
It is desirable for a bushing to be able to be held in angular relation to a transformer casing. This angular configuration is usually achieved by providing a modified transformer casing with a gusset upon which a porcelain bushing is seated. Transformer casings which are modified to include a gusset are expensive and time consuming to construct. Prior art porcelain bushings and assemblies have so far been unable to meet this desire.
Non Ceramic prior art addresses some of these shortcomings, for example the casting of the conductor into the resin system, but does not adequately address the fixing of the bushing to the transformer and sealing of the bushing to the transformer case. Prior art has also explored various metallic clamping devices on the inside or outside of the transformer, all of which detract from the insulative capacity of the bushing.